Selecting an access point according to a measure of received signal quality

ABSTRACT

A method, an apparatus for inclusion in a wireless station, and a computer readable storage medium for operation in a wireless station. The method includes received data from at least one remote station and determining an EVM measure from samples of the received data. If the remote station(s) is/are access point(s), the station selects an access point for association according to criteria that include the measure of the EVM from the remote station.

The present invention is a continuation of U.S. patent application Ser.No. 11/186,045 filed Jul. 20, 2005 to inventors Goodall, et al. U.S.patent application Ser. No. 11/186,045 is a continuation of U.S. patentapplication Ser. No. 10/700,011 filed Nov. 3, 2003 to inventors Goodall,et al., now U.S. Pat. No. 6,940,843. U.S. patent application Ser. No.10/700,011 is a continuation-in-part of U.S. patent application Ser. No.10/367,010 filed Feb. 14, 2003 to inventors Ryan, et al., now U.S. Pat.No. 6,898,198. The contents of each of U.S. patent application Ser. Nos.11/186,045, 10/700,011, and 10/367,010 are incorporated herein byreference.

BACKGROUND

This invention is related to wireless networks. One aspect is a methodfor a wireless station of a wireless network to select an access pointaccording to a received signal quality measure. One version uses ameasure of the error vector magnitude (EVM).

Wireless networks such as wireless local area networks (WLANs) haverecently become popular. A WLAN may be an ad-hoc network in which anywireless station (STA) may communicate directly with any other STA, oran infrastructure network in which one STA acts as an access point (AP).All other STAs of the network associate with the AP, and communicateonly via the AP. The AP may be connected to other networks by a wired orwireless connection.

The description herein will assume a wireless network that conforms tothe IEEE 802.11 standard, and will use the terminology of the IEEE802.11 standard. The invention, however, is not restricted to such anetwork.

A station of a wireless network, i.e., a STA, includes a physical layerprocessor (PHY) and a MAC processor. An AP is a STA that transmitsmessages (beacons or probe responses) that provide information (PHY andMAC information) for other stations that enable such other stations toassociate with the AP. Beacons or probe responses are similar exceptthat a beacon is broadcast not necessarily in response to any externalevent, and a probe response is transmitted in response to the APreceiving a probe request message. For any particular STA that desiresto associate with an AP, there may be several APs with which toassociate. STAs often scan for APs with which they can associate anddesire to associate with the “best” AP.

A STA that wants to operate as a client station in an infrastructurenetwork, also called a basic service set (BSS), will usually attempt toidentify all the APs with which it can associate by scanning one or morechannels, e.g., for beacons and probe responses. The STA will so scan atstart-up and periodically thereafter.

An AP that acts as a repeater AP will also scan one or more channels toidentify potential parent APs, although usually only on start-up or ifits parent AP is no longer available.

The scanning may be active or passive. In a “passive scan”, a STAlistens for beacons from APs on one or more channels. In an “activescan”, a STA sends a probe request message and listens for proberesponses in response to the probe request, on one or more channels.

A STA may also record other packets from an AP in addition to beaconsand probe responses.

Scans provide information at the MAC level such as the data ratessupported by the AP, the identifier of service set (the SSID), securityparameters for communicating with the AP, the load at the AP, and soforth. Scans also provide PHY layer information. In particular, when aSTA receives the beacon or probe response, the STA records the receivedsignal strength indication (RSSI) at the PHY of the receiving STA.

The information provided by scanning, after appropriate weighting, isoften used to determine the “best” AP with which an association shouldbe attempted.

The IEEE 802.11a PHY standard defines RSSI as a measure by the PHYsublayer of the energy observed at the antenna used to receive thepacket. RSSI is measured by the PHY during packet reception and ispassed up with the packet. The RSSI is often used to differentiate thesignal strength from candidate APs and to determine the “best” APaccording to the received signal strength when all other measures, suchas loading, etc., are equal.

Those skilled in the art will recognize that the RSSI is a measure ofsignal strength but not signal quality. It has been found that the RSSIis not a good indicator of the signal quality or a good measure for“best” AP selection. This may be for a variety of reasons. For example,when scanning, often the only packets available for a STA to measure aresent at a relatively low rate. This is particularly true if RSSI valuesfor AP selection are only available from a beacon or a probe response,which are usually sent at a low rate. The RSSI value from a low ratepacket only provides a very coarse indication of how well the link willsupport high rate packets. Such an RSSI value, for example, does notaccount for factors that significantly reduce signal quality such asmultipath or the presence of strong interferers.

Using the RSSI to select the “best” AP can result in a lower throughputand latency than would occur if a measure of signal quality rather thansignal strength was used for the selection. Selecting the AP using theRSSI can also lead to frequent roaming, and hence instability. These areparticular issues for voice and other applications that require highthroughput or low latency.

Thus there is a need in the art for a method of selecting an AP forassociation based on a measure indicative of the received signal qualityand of the quality of communication achievable on the link between theAP and a client station.

SUMMARY

Described herein is a method for operation in a wireless station, themethod including wirelessly receiving a signal from a remote stationcorresponding to a packet transmitted by the remote station,demodulating samples of the received signal to produce demodulatedsignals from the remote station, and selecting whether or not tocommunicate with the remote station based on a measure of the receivedsignal quality, e.g., the received signal EVM of data transported viathe link between the station and the remote station.

In one application, the remote station is an access point, and theselecting is whether or not to associate with the remote access point.In another application, the remote station is a station and theselecting is to select whether or not to communicate with the remotestation on an ad-hoc basis.

In one embodiment, the transmitted packet corresponding to the receivedsignal includes the measure of the EVM obtained from samples of signalsreceived by the remote station corresponding to data transmitted by thestation, such that the station need not be capable of determining theEVM

In another embodiment, the method further includes determining at thestation the measure of the received signal EVM from samples of thereceived signal. According to an improvement, wherein the transmittedpacket corresponding to the received signal includes a measure of thereceived signal EVM obtained from samples of signals received by theremote station corresponding to data transmitted by the station, suchthat the selecting by the station uses measures of the quality of datatransmitted both directions via the link between the station and theremote station

Also described herein is an apparatus embodiment for inclusion in astation of a wireless network. The apparatus includes a radio receiverto wirelessly receive data from at least one remote station. The data istransmitted by the remote station as at least one packet of data. Thereceiver includes an analog-to-digital converter producing samples ofsignals received at the station from the remote station. The apparatusincludes a demodulator coupled to the radio receiver to demodulatesamples of the signals received at the receiver from each station toproduce demodulated signals from each of the remote stations. Theapparatus further includes a signal quality calculator coupled to thereceiver to determine for each remote station from which data isreceived a measure of the received signal quality based on the samplesof the received data from the remote station, and a transmitter totransmit data for transmission.

In one version, the signal quality measure is a measure of the EVM.

In one aspect of the invention, in the case that the received data isascertained to include a request management message, the stationresponds to the request management message with a response managementthat include a measure of the EVM of received data corresponding torequest management message. Thus, the remote station receiving theresponse management message receives an indication of the quality of thelink between the station and the remote station without said receivingremote station necessarily being EVM-capable.

In one version, the wireless network substantially conforms to the IEEE802.11 wireless networking standard. The request management message andthe response management message are MAC frames.

In another aspect of the invention, a message to a particular remotestation in response to the data received at the station from theparticular remote station includes a measure of the EVM of the datareceived from the particular remote station. In one version, thewireless network substantially conforms to a wireless networkingstandard, e.g., one of the of the OFDM variants of the IEEE 802.11standard, and a packet according to the wireless networking standardincludes a header having a first field modulated at a known rate. Themessage to the particular station includes the measure of the EVM in thefirst field.

According to one version of the invention, the station selects one ofthe remote stations for communication according to a set of at least onecriterion. The set includes the respective received signal qualitymeasure determined by the signal quality calculator for data from eachof the respective remote stations.

One application is when the selecting of one of the remote stations isfor ad-hoc communication.

According to another application of the invention, in the case at leastsome of the remote stations from which data are received are accesspoints, the station selects one of the access points for associationaccording to a set of at least one criterion. The set includes therespective received signal quality measure determined by the signalquality calculator for data from each of the respective access points.

In one version, the signal quality calculator is an EVM calculator todetermine for each remote station from which data is received a measureof the EVM of the received data from the remote station. The EVM isbased on samples approximately at decision points of the demodulator. Ina particular embodiment, the data received from remote stations that areaccess points include beacons or probe responses, such that one of thecriteria for the station to select a remote access point for associationis a measure of the EVM of a beacon or probe response received from theaccess point.

In another embodiment, one of the remote stations from which data isreceived is an access point with which the station is associated, suchthat the station decides whether or not to roam according to the measureof signal quality from data received from the remote access point withwhich the station is associated.

According to another aspect, the EVM of a beacon or probe responsereceived from a remote access point is used to determine the maximumtransmission rate that the link can support between the station and theremote access points, and wherein the determined maximum supportedtransmission rate is one of the criteria for the station to select aremote access point for association.

According to another aspect, the receiving station is an access pointand in the case that the received data is ascertained to be a proberequest, an association request or a reassociation request, a packet fortransmission by the transmitter from the station to a particular remotestation that sent the request includes a measure of the received signalquality of the request received from the particular remote station.

According to another aspect, the transmitter has a settable data rate,with the data rate set according to a data rate signal accepted by thetransmitter. The apparatus further includes a data rate settingprocessor coupled to the signal quality calculator and to thetransmitter and producing the data rate signal for the transmitter, thedata rate signal set such that the data rate for transmission to aparticular remote station is dependent on the measure of the signalquality produced by the signal quality calculator from signals receivedfrom the particular remote station.

Also described herein is a method embodiment in a station of a wirelessnetwork. The method includes wirelessly receiving data from at least oneremote station. The data is transmitted by the remote station as atleast one packet of data. The method further includes sampling thereceived data corresponding to the received packet to form data samples,demodulating the data samples, and determining a measure of signalquality from the samples of the received data.

According to one aspect, in the case that at least some of the remotestations are access points, the method includes selecting one of theaccess points for association according to a set of least one criteriaincluding the respective determined received signal quality measure fordata from each of the access points.

In one version, the determining of the measure of signal qualityincludes determining a measure of the EVM of the received data fromreceived samples approximately at the decision points for demodulatingthe data.

According to another aspect, the method further includes selecting thedata rate for communicating with the selected access point according toat least the determined measure of the EVM.

According to another aspect, the station is an access point, and themethod further includes, in the case that the received data isascertained to be a probe request or an association request forassociation or re-association, transmitting a packet to the particularremote station that sent the probe request or association request, thetransmitted packet including an indication of the determined measure ofthe EVM of the packet received from the particular remote station.

According to another aspect, the method includes ascertaining whetherthe received data from any remote station is a request managementmessage. In the case it is ascertained that a request management messagewas received from a particular remote station, the method includesresponding to the request management message by transmitting to theparticular remote station a response management message that include ameasure of the EVM of received data corresponding to request managementmessage. The particular remote station receiving the response managementmessage receives an indication of the quality of the link between thestation and the remote station without said receiving remote stationnecessarily being EVM-capable.

Another method embodiment described herein is a method in a station of awireless network. The method includes wirelessly transmitting a requestmanagement message, wirelessly receiving data from at least one remotestation, said data transmitted by the remote station as at least onepacket of data, sampling the received data corresponding to the receivedpacket to form data samples, demodulating the data samples, andascertaining if the received data includes a response management messagetransmitted in response to a request management message.

If it is ascertained that the received data includes the responsemanagement message, the method ascertains if a packet has been receivedfrom the particular remote station that transmitted the responsemanagement message indicating a measure of the signal quality of thedata remotely received at the particular remote station corresponding tothe transmitted request management message. The message indicating themeasure of signal quality can be used as an indication of the quality ofcommunication achievable between the station and the particular remotestation.

According to another aspect, if it is ascertained that such a packet hasbeen received from the particular remote station, the method includesselecting whether or not to associate with the particular remote stationaccording to at least the measure of the signal quality of the remotelyreceived data.

In one version of the method, the determining of the measure of signalquality includes determining a measure of the EVM of the received datafrom received samples approximately at the decision points fordemodulating the data.

In one embodiment, the determining of the measure of the EVM of thereceived packet includes determining the average of the squaredEuclidian distance on the I,Q plane between decision-point samples ofthe signal received and the nearest ideal constellation points to thedecision point samples.

In another embodiment, the determining of the measure of the EVM of thereceived packet includes determining the average of the squaredEuclidian distance on the I,Q plane between decision-point samples ofthe signal received and the correct ideal constellation points for thesignal as determined by demodulating the signal.

Other aspects of the invention will be clear from the detaileddescription, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an I,Q plane with a constellation of four possible symbolsS₀, S₁, S₂, and S₃ for a signal that is modulated by a quaternary phaseshift key (QPSK) modulation scheme.

FIG. 2 is a functional block diagram of a wireless station that includesan embodiment of present invention.

FIG. 3 shows a simplified block diagram of the OFDM receiver part of themodem shown in FIG. 2 according to one embodiment of the presentinvention.

FIG. 4 shows the order of data transmitted to the MAC processor in thecase that the data is OFDM data and that PPDU mode is enabled, accordingto one embodiment of the invention.

FIG. 5 shows a flow chart of an embodiment of a method implemented on anEVM-capable station of selecting an access point for association.

FIG. 6 shows a flow chart of an embodiment of a method implemented on anEVM-capable access point receiving a probe request and responding with aprobe response that includes a measure of the EVM of the received proberequest.

FIG. 7 shows a flow chart of an embodiment of a method implemented on anEVM-capable station sending a probe request to an access point andreceiving a probe response.

DETAILED DESCRIPTION

The invention is applicable to ad-hoc network configuration in which anystation can communicate directly with another station, and to aninfrastructure network configuration in which client stationscommunicate via an access point with which they are associated.

Described herein is a method for a station to select a station forcommunication, e.g., an access point for association in theinfrastructure network case, using a measure of received signal qualityto indicate of the data rate that the link between the station and thecandidate AP can support. One embodiment includes calculating thereceived signal quality measure locally, and another includes measuringthe received signal quality measure remotely.

One embodiment of the method uses a measure of the Error VectorMagnitude (EVM) on packets received from the candidate access point—orin the ad-hoc communication case, from the candidate partner forcommunication—to provide an indicator of signal quality. Anotherembodiment uses a measure of the EVM of packets from the stationreceived at the candidate AP or communication partner.

The invention will be described in terms of a transceiver that operatedaccording to OFDM variants of the IEEE 802.11 standard, includingvariants commonly known as 802.11a (approx. 5 GHz UNII band) and 802.11g(approx. 2.4 GHz) that support several data rates up to 54 Mbps, as wellas future or proposed amendments to the IEEE 802.11 standard, such aswhat is being called the IEEE 802.11n variant for high throughput beingstudied (late 2003) by IEEE 802.11 Task Group N. The invention is alsoapplicable to a radio that operated under any other wireless standard,including other variants in the IEEE 802.11 standard.

EVM is a modulation quality metric widely used in digital RFcommunications systems. It is typically defined for a transmitter or areceiver, and is the root-mean-square (RMS) value of the error vectorover time for the signal at the correct symbol time. In the case of areceiver, the correct symbol times are approximated by the decisionpoints—the times when demodulation decisions are made. The error vectoris the vector length of the difference, in the complex plane (I,Q space)between an ideal constellation point of a symbol, and the actualconstellation point of a symbol at the symbol time for the signal.Consider, for example, FIG. 1 that shows the constellation of the fourpossible symbols S₀, S₁, S₂, and S₃ for a signal that is modulated by aquaternary phase shift key (QPSK) modulation scheme. Ideally, eachactual symbol at the symbol time falls on one of the four constellationpoints. In practice, because of phase error and/or magnitude error, theI,Q values for the modulated signal samples fall on points that are notexactly at the four ideal constellation points. FIG. 1 shows many suchI,Q values, each indicated by an “X.” Two such signal samples are shownas 103 and 105 near the S₀ constellation point 101. Consider point 103.The EVM is the mean of the square root of the square of thelengths—e.g., length 107—of the vectors that are the errors between thecorrect constellation point—e.g., constellation point 101—and the actualsignal point—e.g., point 103. This quantity is normalized by the signalstrength, so is expressed as a percentage (% RMS). Alternatively, it maybe expressed in dB. When expressed in dB, the EVM is sometimes referredto as the relative constellation value.

Thus, a measure of the EVM in general varies monotonically with ameasure of the distance squared between received symbols and idealsymbols, divided by the distance squared from ideal symbols to zero.

The measurement of EVM is somewhat complicated by carrier leakage thatcauses the origin of the IQ axes to shift.

One aspect of the invention is to use the EVM as a measure of thequality of the received signal. Used properly, EVM and relatedmeasurements can pinpoint exactly the type of degradations present in asignal and can even help identify their sources.

FIG. 2 is a functional block diagram of a wireless station 200 thatincludes an embodiment of present invention. The station 200 may, forexample, implement an AP or may implement a client station that desiresto associate with an AP. The station 200 includes a physical layerinterface (PHY) 201 that includes at least one antenna 202 for thefrequency or frequencies of service (approx. 2 GHz and/or approx. 5GHz), a transmit/receive (T/R) switch 204 for half-duplex operation, awireless receiver that includes a low-noise amplifier (LNA) 206 andreceiver radio frequency (RF) electronics 210, and a wirelesstransmitter that includes transmit RF electronics 211 and a poweramplifier (PA) 208. The PHY also includes a modem 207 that includes ademodulator 212 and a modulator 213. The receive path to the demodulatorincludes an analog-to-digital converter (ADC) 227 to produce samples ofthe received signal. The system 200 further includes a medium accesscontroller (MAC processor, or simply MAC) 214 for layer-2 processing. Acomputer system databus 218 is included in one embodiment, as is a hostprocessor 215.

In one embodiment, a memory 216 is included for data buffering andprogram storage. The memory 216 may be directly coupled to the host orto the MAC or to both.

Alternate embodiments do not include the host processor. The hostprocessor function may, for example, be incorporated with the MAC 214.

In addition to the payload data between the modem 207, radio transceiver205, and MAC 214, control information such as gain settings for theradio receiver from an automatic gain control module in the modem 207,and other data is communicated between the transceiver and the modem.Furthermore, other data may be communicated between the modem and theMAC, and between the modem and the host (if included).

A set of registers 237 is included. In one embodiment, the MAC processor214 has access to at least some of the set of registers 237. Someregisters can be loaded with data from the MAC 214, others provide datafor the MAC processors, while some may provide for two-waycommunication.

In one embodiment, the modem 207 includes a signal quality calculator229 that determines a measure of the received signal quality fromsamples of the received signal. One embodiment of the signal qualitycalculator 229 is an EVM calculator that determines a measure of the EVMfor a received packet or part thereof, and communicates the EVM measureto the MAC.

In one embodiment, the EVM is provided in a pair of registers of theregister set 237. The pair of registers is used for the numerator anddenominator, respectively, of the determined EVM measure.

Some embodiments may use antenna diversity, e.g., two or more transmitantennas or two or more receive antennas or multiple antennas for bothreceiving and transmitting. The diversity may be provided by spatialdiversity, or by having different polarizations at the antennas, and soforth. The antennas may be switched or combined. Such processing isknown to improve performance in environments that include fading, andmay even be used to provide spatial division multiple access (SDMA).

One embodiment of system 200 is compatible with one or more variants ofthe IEEE-802.11 standards for wireless local area network (WLAN)applications. The RF transceiver 205 and modem 207 constitute a completewireless engine for layer-1 physical layer (PHY) functionality for oneor more of the IEEE-802.11 PHY variants, and the MAC 214 isIEEE-802.11-compatible.

One embodiment of the system 200 preferably is constructed on a singleprinted circuit board (PCB). The RF transceiver 205 and modem 207 areeach implemented with CMOS technology in individual integrated circuits(chips).

The OFDM Receiver

One embodiment of the invention is applicable to communicating usingOFDM packets that conform to the IEEE 802.11 OFDM variants. Such an OFDMpacket starts with a preamble. The preamble provides for start of packet(SOP) detection, automatic gain control (AGC), diversity selection whendiversity is used, various other synchronization functions, and channelestimation. The preamble is followed by the modulated payload, whichstarts with a known (low) data rate SIGNAL field that providesinformation about the packet, including the data rate at which the restof the packet in encoded. The SIGNAL field is followed by DATA fields ata rate specified in the SIGNAL field. Each data field includes a guardinterval (cyclic extension). The SIGNAL field includes information onthe data rate (RATE). The RATE information determines the coding rateand the modulation method used according to the following Table 1:

TABLE 1 Modulation type for IEEE 802.11 OFDM packets Rate (Mbps)Modulation type Coding rate 6 BPSK 1/2 9 BPSK 3/4 12 QPSK 1/2 18 QPSK3/4 24 16-QAM 1/2 36 16-QAM 3/4 48 64-QAM 2/3 54 64-QAM 3/4

One embodiment further includes several proprietary modulationadditional data rates that do not conform to the IEEE 802.11 standard.

FIG. 3 shows a simplified block diagram of the OFDM receiver part of themodem 207. The embodiment shown works with a version of the receive RFelectronics 210 that produces an IF signal with a 20 MHz bandwidthrequiring further down-conversion to obtain baseband I,Q signals. Theanalog IF signal from the receive RF electronics 210 may be set to becentered at a settable frequency between 20 MHz and 25 MHz, e.g., at 20MHz. An ADC 227 digitizes the analog received signals. Element 309further down-converts and decimates the signals to produce I and Qsamples that enter a first in first out (FIFO) buffer 310 and a timesynchronization unit 311. Synchronization is achieved by one or moremethods, such as estimating the short preamble's short symbol timingusing a correlator and estimating the guard interval timing of the longpreamble. The synchronization unit further includes frequency estimationand frequency correction using a rotator. The output of the rotator ispresented to a 64-sample input buffer 312 of a 64-sample discreteFourier transformer 313 (FFT64) for time-to-frequency conversion.

The Fourier transformer 313 transforms the complex baseband time-domainsamples of the OFDM symbols of the received packet into complex-valuedfrequency domain samples by a Discrete Fourier Transform (DFT)operation. The IQ frequency domain samples from Fourier transformer 313,in bit-reversed frequency order, are converted to polar coordinates by arectangular-to-polar (amplitude/phase) converter 314—a cordic in oneembodiment. The result is input into a channel responseestimator/equalizer block 315 that estimates the channel response andthat equalizes the signals according to the estimated channel response.Also included in estimator/equalizer block 315 are a channel stateinformation detection circuit and a pilot tone correction unit. Thecorrected signals are converted back to I,Q (rectangular coordinate)form by a second coordinate converter 316. The output is thus a sequenceof IQ frequency samples ready for demodulation.

A rate buffer 317 is included between the second coordinate converter316 and a demodulator 319. The rate buffer 317 is a second FIFO whichbuffers the received frequency samples from the end of the SIGNAL fieldfor a number clock cycles equal to the latency (in clock cycles) throughthe later parts of the receiver needed for processing the SIGNAL field,including demodulating and decoding. The rate buffer block 317 isincluded because the SIGNAL field, which is always transmitted as a 6Mbps rate ½ binary phase shift key (BPSK) signal, determines themodulation type and coding rate of the remainder of the packet, shown asRATE in FIG. 3. This information is then used to set up the demodulatorand Viterbi decoder parameters before the remainder of the packet isinput into the demodulator chain. Hence, the remainder of the packetneeds to be buffered until at least the RATE information has beensuccessfully decoded.

The output samples (I,Q) of the rate buffer are demodulated by thedemodulator 319. In one embodiment, the samples are first rounded from12-bits for each of I and Q to 6 bits for each by a rounder 318. Thedemodulator 319 demodulates depending on the modulation specified by theRATE. The demodulated symbols from demodulator 319 are de-interleaved byinterleaver 325 and symbols inserted in symbol inserter 327 to ensurethe symbols have the coding rate to match the decoder included in thisone embodiment. The output symbols of symbol inserter 327 are input to aViterbi decoder 331 and descrambled by descrambler 333. Thede-interleaving, symbol insertion, and Viterbi decoding depend on theRATE. The series of descrambled symbols are converted to parallel formby serial-to-parallel converter 335 for input to the MAC processor.

Initially, the demodulator, de-interleaver, decoder, etc. are set toprocess the SIGNAL field. Once the RATE information, including themodulation scheme, is determined, these elements are set to demodulatethe data frames of the payload. In this embodiment, the Viterbi decoder331 is a ½-rate decoder. The symbol inserter 327 is included to insertdummy signals to convert non-½ rate data into data suitable for theViterbi decoder 331.

The receiver generates the received data of a packet and passes the datato the MAC processor 214. Additional information also is passed on tothe MAC layer processor, including information about the packet. In oneembodiment, such information includes a measure of the EVM of the SIGNALpart, a measure of the EVM of the data part of the packet, and the RSSI.

The receiver includes an EVM calculator 229 coupled to and acceptinginput from the rate buffer, i.e., accepting I,Q samples at the decisionpoints. The EVM calculator determines a measure of the EVM of the OFDMsymbols in the packet. The EVM determination depends on the idealconstellation points for the particular modulation type. Hence, EVMcalculator 307 also accepts RATE information.

A measure of the EVM is calculated by EVM calculator 229 and passed onto the MAC layer processor 214. In one embodiment, the measure is passedto the MAC as a numerator and denominator. In one embodiment, once theSIGNAL field is processed, data is passed on to the MAC processor forfurther processing according to the appropriate MAC protocol.

In one embodiment, the EVM measure for the SIGNAL part and the EVMmeasure for the data part of each arriving packet are separatelycommunicated to the MAC processor.

We call a station that includes an EVM calculator an EVM-capablestation. According to an aspect of the invention, a station uses ameasure of the EVM as a measure of the signal quality as one of thecriteria for selecting an AP for association or, in the ad-hoccommunication case, another station—the communication partner—forcommunication. An aspect of the invention is embodied in an EVM-capablestation as a method for that station to select a remote,not-necessarily-EVM-capable station for communication, e.g., AP forassociation. Another aspect of the invention is embodied as a method inan EVM-capable station that sends EVM information to anothernot-necessarily-EVM-capable station. Another aspect of the invention isa method embodied in a not-necessarily-EVM-capable station that receivesthe EVM data from an EVM-capable station.

Another aspect of the invention is that the EVM is used to provideinformation to the transmitter part of the modem 207 on what rate to useto send data to the station from which the data is being received.

One definition of the EVM of an OFDM packet (or parts of a packet) oflength L_(p) using 52 sub-carriers (including pilot tones) is:

${EVM}_{RMS} = \sqrt{\frac{\sum\limits_{j = 1}^{L_{p}}\left\{ {\sum\limits_{k = 1}^{52}\left\lbrack {\left( {{I\left( {j,k} \right)} - {I_{0}\left( {j,k} \right)}} \right)^{2} + \left( {{Q\left( {j,k} \right)} - {Q_{0}\left( {j,k} \right)}} \right)^{2}} \right\rbrack} \right\}}{52L_{p} \times P_{0}}}$

where I₀(j,k) and Q₀(j,k) denote I,Q for an ideal symbol point of thej'th OFDM symbol of the packet, and k'th sub-carrier of the symbol inthe complex I,Q plane, I(j,k) and Q(j,k) denote the received I,Q valuesof the j'th OFDM symbol of the packet, and k'th sub-carrier of thesymbol in the complex I,Q plane, and P₀ denotes the average power of theconstellation.

The EVM calculator 229 is used to calculate a measure of the EVM, e.g.,a function of the calculated EVM, EVM_(calc) according to the followingformula:

${f\left( {EVM}_{calc} \right)} = \frac{\sum\limits_{j = 1}^{L_{p}}\left\{ {\sum\limits_{k = 1}^{48}\left\lbrack {\left( {{I\left( {j,k} \right)} - {I_{n}\left( {j,k} \right)}} \right)^{2} + \left( {{Q\left( {j,k} \right)} - {Q_{n}\left( {j,k} \right)}} \right)^{2}} \right\rbrack} \right\}}{48L_{p} \times P_{0}}$

where there are 48 subcarriers—the 52 subcarriers but without the pilottones, f is the function of the EVM determined and I_(n)(j,k) andQ_(n)(j,k) denote the I,Q values of the nearest for an ideal symbolpoint to the actual I,Q values of the j'th OFDM symbol of the packet,and k'th subcarrier of the symbol in the complex I,Q plane. In oneembodiment, the numerator and denominator of f(EVM_(calc)) as definedabove are determined by the EVM calculator 229 and passed to the MACprocessor for determination of f(EVM_(calc)), the measure of the EVM. Inone embodiment, f(EVM_(calc)) is the square of EVM_(calc), thecalculated EVM. The EVM calculator 229 determines the nearest neighbordecision point using a hard decoder. Because of the use of a harddecoder, using this measure of the EVM may lead to errors, e.g., havinga higher signal quality than the actual signal quality. Such errors areespecially possible when the actual EVM is large, e.g., at high datarates when the quality of the signal is relatively low. Determining theEVM according to the Euclidean distance in the I,Q plane to the nearestideal constellation point is however less complex than determining thedistance to the correct ideal constellation point.

In an improved embodiment, the EVM calculator is also coupled to thedemodulator and determined a measure closer to the true EVM than thenearest neighbor embodiment. Once the signal is demodulated, the idealI,Q values for the demodulated signal, e.g., of a reference signalmodulated by the decision points, are determined and the measure of EVMdetermined by EVM calculator 229 is according to the Euclidean distancein the I,Q plane to the correct ideal constellation point according tothe demodulation. In particular, the improved embodiment EVM calculatordetermines:

${f\left( {EVM}_{calc\_ improved} \right)} = \frac{\sum\limits_{j = 1}^{L_{p}}\left\{ {\sum\limits_{k = 1}^{48}\left\lbrack {\left( {{I\left( {j,k} \right)} - {I_{0}\left( {j,k} \right)}} \right)^{2} + \left( {{Q\left( {j,k} \right)} - {Q_{0}\left( {j,k} \right)}} \right)^{2}} \right\rbrack} \right\}}{48L_{p} \times P_{0}}$

where again there are 48 subcarriers—the 52 subcarriers but without thepilot tones, f(EVM_(calc) _(—) _(improved)) is the function of thecalculated EVM according to the improved method, and I₀(j,k) and Q₀(j,k)denote the I,Q values of the ideal symbol point.

Selecting Another Station for Communication, e.g., an AP for Association

According to one aspect of the invention, each STA such as STA 200maintains a database of the beacons and probe responses it receives.Beacons and probe responses are stored in the database under one or morecircumstances, e.g., when the STA determines whether or not to associatewith an AP. In the context of aspects of the present invention, beaconsand probe responses received at the STA are stored in the database afteran active scan or a passive scan. We call this database the BeaconTable. As shown in FIG. 2, in one embodiment, the Beacon Table 231 is inthe memory 216 of the STA. Other embodiments store the Beacon Table 231outside of memory 216. A STA stores the information in the beacons andprobe responses in its Beacon Table 231, and further stores additionalinformation about the state of the STA when it receives the beacon.

Another embodiment described below, in addition to information frombeacons and probe responses, information on any packets from remotestations, even those directed to other, third-party stations isrecorded. See below for the case that the station is EVM-capable andmaintains information, including the EVM of received packets from anystation it can hear.

Selection by an EVM-Capable Station

By an EVM-capable station is meant a station that includes a signalquality calculator in its PHY processor that calculates the quality of areceived signal. One implementation of such an EVM-capable station isSTA 200 of FIG. 2, wherein the signal quality calculator calculates theEVM of data received from a remote station.

One aspect of the invention is an EVM-capable station that selects aremote station for communication, e.g., access point for association oranother station for ad-hoc communication, according to a measure of thesignal quality, in particular, of the EVM of data received from theremote station, e.g., remote access point. Thus, one embodiment is amethod in an EVM-capable station of a wireless network. FIG. 5 shows asimplified flowchart of the method 500. The method includes (step 501)wirelessly receiving data from at least one remote station, the datatransmitted by the remote station in packets, sampling the received datacorresponding to the received packet to form data samples, demodulatingthe data samples, and determining (step 503) the EVM from the samples ofthe received data.

In the case that at least some of the remote stations are access points,the method includes (step 505) selecting one of the access points forassociation according to a plurality of criteria, including therespective determined received signal quality measure for data from eachof the access points.

In the case that the candidate remote stations are for ad-hoccommunication, the method includes (step 505) selecting one of theaccess points for communication according to a plurality of criteria,including the respective determined received signal quality measure fordata from each of the candidate remote stations.

The remote station, e.g., access point need not itself be EVM-capable.

In one embodiment, the packets received from the remote access pointsare beacons or probe responses, such that one of the criteria for thestation to select a remote access point for association is a measure ofthe EVM of a beacon or probe response received from the access point.

The EVM of received data corresponding to at least part of the beacon orprobe response received from a remote access point is used to determinethe maximum transmission rate that the link can support between thestation and the remote access points. Thus, the determined maximumsupported transmission rate is one of the criteria for the station toselect a remote access point for association.

Another embodiment uses the EVM of a response to an association orreassociation request to determine whether to roam to another accesspoint. An EVM-capable station selects an access point for association orreassociation and issues an association or reassociation requestmessage, e.g., an association or reassociation request MAC frame. Theremote access point responds with an association or reassociationresponse message, e.g., an association or reassociation MAC frame. TheEVM of signals corresponding to data in the association or reassociationresponse message received from the remote access point is used todetermine whether or not to seek another access point for association.

According to yet another embodiment, the EVM from any frame received byan EVM capable station from a remote access point with which the stationis associated determines a roam.

According to one aspect of the invention, each STA such as STA 200maintains a database of the packets it receives from any remote station.In the case the packets are beacons and probe responses it receives, thebeacons and probe responses are stored in an AP database. Oneembodiment, in addition to information from beacons and probe responses,stores information on any packets from remote stations, even thosedirected to other, third-party stations is recorded. One item ofinformation stored is the EVM of received packets from any station itcan hear. Such information can then be used for selecting a database forassociation or reassociation.

Selection by a Not-Necessarily-EVM-Capable Station

Another aspect of the invention is a station that is not necessarilyEVM-capable selecting a remote access point for association using ameasure of the EVM of data received via the link.

One embodiment of the method implemented at the not-necessarilyEVM-capable station includes the not-necessarily EVM-capable stationreceiving a measure of the EVM of data received via the link from aremote access point that is EVM-capable. FIG. 6 shows a embodiment 600of the method implemented at an EVM-capable station, such as an accesspoint or a potential candidate for ad-hoc communication. The embodimentincludes the EVM-capable station—e.g., access point—receiving a requestmessage, e.g., a message from a remote station requesting association orreassociation, or a probe request message from a remote station, andresponding to the received request message with a response message thatincludes an indication of the received signal quality from samples ofthe request message. The case shown in FIG. 6 is that in step 615, arequest is received. In step 617, the EVM-capable station calculates ameasure of the EVM from the request, and in step 619, the EVM-capablestation responds to the sending station with a response that includesthe determined EVM.

The not-necessarily EVM-capable station selects whether or not tocommunicate with the remote EVM-capable station, e.g., whether or not toassociate with the remote access point or whether or not to communicatewith the remote EVM-capable station on an ad-hoc basis, according to thereceived measure of signal quality, e.g., the EVM of the link sent bythe remote EVM-capable station. Such selection, for example, may includeselecting from a plurality of available remote stations, e.g., remoteaccess points.

Another aspect of the invention includes modifying the existing MACframe structure to include the EVM measure in a probe response, anassociation response, and a reassociation response MAC frame so that anEVM-capable station can send a measure of the EVM to a remote stationfrom which the EVM-capable station received a signal. The modified MACstructure includes an indication of whether or not the frame includes anindication of the EVM so that a station receiving the frame canascertain whether or not the received frame is from an EVM-capablestation, and thus includes a measure of the EVM.

Thus, in the infrastructure network application, a not necessarilyEVM-capable station receiving a beacon from a remote access point cansend out a probe request and receive in response a probe response fromthe remote access point. In the case that the remote access point isEVM-capable, the probe response includes a measure of the EVM of datafrom that station remotely received at the remote access point. Thus,one aspect includes the station ascertaining if the probe response isfrom an EVM-capable station, and thus would include the measure of theEVM. The station then selects whether or not to associate with theremote access point according to the received measure of the EVM. Inpractice, the station may select from a set of remote access points fromwhich the station has received such EVM measures.

In an alternate implementation, again for the infrastructure networkapplication, a not necessarily EVM-capable station receiving a beaconfrom a remote access point can send out an association request andreceive in response an association response from the remote accesspoint. In the case that the remote access point is EVM-capable, theassociation response includes a measure of the EVM of data from thatstation remotely received at the remote access point. Thus, one aspectincludes the station ascertaining if the association response is from anEVM-capable station, and thus would include the measure of the EVM. Thestation then selects whether or not to reassociate with a differentstation in the case EVM measure is below a settable threshold, or in thecase that the EVM measure is significantly lower than that from one ormore other remote access points from which the station has received suchEVM measures.

An alternate implementation introduces new MAC management frames thatare measurement frames. One such frame is a measurement request MACframe. An EVM-capable station responds to a measurement request MACframe with a response management MAC frame that includes an indicationof the signal quality, e.g., the EVM of the signal corresponding to atleast part of the measurement request MAC frame.

Thus, in the case of an infrastructure network, a station sends out ameasurement request MAC frame to an AP, e.g., one that responded to aprobe request with a probe response MAC frameAn EVM-capable stationresponds to the measurement request MAC frame with a response managementMAC frame that includes an indication of the signal quality, e.g., theEVM of the signal corresponding to at least part of the measurementrequest MAC frame. The station may now select whether or not toassociate with the remote AP based on an indication of the receivedsignal quality at the AP.

Note that while the above describes the case of a not-necessarilyEVM-capable station deciding whether or not to associate with anEVM-capable access point, the method is equally applicable to the caseof a not-necessarily EVM-capable station deciding whether or not toselect an EVM-capable remote station—not necessarily an access point—forcommunication, e.g., on an ad-hoc basis. Those in the art will be ableto modify the method described above for the case of the not-necessarilyEVM-capable station receiving a packet from an EV-capable remote stationthat includes a measure of the signal quality, e.g., of the EVM, of datareceived a the remote station from the not-necessarily EVM-capablestation.

A Two Way Measure

It may be that the link between a station and a remote station is notperfectly symmetrical, such that a measure of the signal quality ofpackets received from a remote station would not be the same as anidentically determined measure of the signal quality of packets receivedat the remote station. For example, the receivers or transmitters ateach end of the link may be different, e.g., have substantiallydifferent specification and capabilities.

An improved embodiment of the invention is applicable when both thestation selecting an access point for association and the remote accesspoint are EVM-capable. The selecting is according to a measure of theEVM of data received via the link. In the improved embodiment, a two-waymeasure is used. Thus, an EVM-capable station received a measure of theEVM of signals from the station received at a remote access point, forexample, using the MAC frames described above. The station further usesits own EVM calculator to determine the EVM of data received from theremote access point. The station now selects whether or not to associatewith the remote access point according to the two EVM measures in eachdirection of the link between the station and the remote access point.

FIG. 7 shows a flow chart of an embodiment 700 of the method, includinga station sending (step 707) a probe request to a remote AP. The AP isEVM capable and responds (see FIG. 6) with a probe response thatincludes a measure of the EVM from the received probe request. Thestation in a step 709 receives the probe response that includes the EVMmeasure. In a step 711, the station determines a measure of the EVM ofsignals corresponding to the received probe response. In a step 713, thestation selects whether or not to associate with the AP (or to associatewith a different AP) based on the EVM in both directions of the link.

Again, while the above shows the case of a station selects whether ornot to associate with an AP, the invention is also applicable to thecase of a station deciding whether or not to communicate on an ad-hocbases with another station.

Response by an EVM-Capable Station

According to one aspect of the invention, an EVM-capable stationresponds to management MAC frames that are requests with responsemanagement MAC frames that include EVM information as well an indicationthat such EVM information is included. The EVM-capable stationcommunicates information about the received requests using MAC framesthat conform to a modification of the IEEE 802.11 standard MAC protocol.For example, according to an aspect of the invention, a standard MACframe used for responding to an association request or a reassociationrequest, i.e., an association response or reassociation response frame,includes in addition to information specified in the IEEE 802.11standard, information on the EVM of the received association request orreassociation request, and an indication that the association responseor reassociation response frame contains such EVM information.Furthermore, according to another aspect of the invention, a standardMAC frame used for responding to a probe request, i.e., a probe responseframe includes in addition to information specified in the IEEE 802.11standard, information on the EVM of the received probe request and anindication that the probe response frame contains such EVM information.

An alternate implementation introduces new MAC management frames thatare measurement frames as described above. One such frame is ameasurement request MAC frame. An EVM-capable station responds to ameasurement request MAC frame with a response management MAC frame thatincludes an indication of the signal quality, e.g., the EVM of thesignal corresponding to at least part of the measurement request MACframe.

Acceptance of the EVM Information by the MAC in an EVM-Capable Station

Referring again to FIG. 2, the modem 207 forms the data for the MACprocessor 214. One embodiment of the modem 207 operates in two modes wecall PSDU (for PLCP Service Data Unit) mode (PCLP is the physical layerconvergence protocol) and PPDU (for PLCP Protocol Data Unit) mode,respectively. In PSDU mode, only packet payload data is transferred tothe MAC processor 214. All receive packet header information isavailable to the MAC processor 214 in the register set 237 of the modem207. In the case of an EVM-capable station at least one of the registerset 237 includes the EVM of data received. In PPDU mode, the PPDU data,such as the PLCP header data, is also sent to the MAC processor 214, andsuch data is sent to the MAC processor once it is decoded.

FIG. 4 shows the order of data transmitted to the MAC processor 214 inthe case that the data is OFDM data and that PPDU mode is enabled. Inone embodiment, a total of nine bytes 400 are sent before the PSDU data.The byte 403 provides the received power at the receiver for the packet,in particular, the received signal strength indication—the RSSI—at thereceiver of transceiver 205 for the packet. The next byte, byte 405provides a measure of the signal quality for the SIGNAL field. In oneembodiment, this is the EVM calculated by EVM calculator 229. The nextbyte contains an indication of the antenna used, the standard (802.11a,gOFDM or 802.11b DSSS/CCK), and other information related to the DSSS/CCKcase. One embodiment also provides for sending some additionalinformation after the PSDU data, e.g., for debugging. The next (fourth)byte 407 indicates to the MAC the number of post PSDU bytes that are tobe included. This is followed by the five-byte PLCP header 409 itself(for the OFDM case). The PSDU data follows.

In one embodiment, the MAC processor retrieves information contained ineach received packet identifying the particular remote station thepacket is from and maintains two metrics for each remote station incommunication with the station. The two metrics maintained at the MACare the EVM measure of the last packet from the remote station, and arunning average of the EVM measures over a number, say the last Npackets from that remote station. For example, in the case of an accesspoint, the MAC maintains two metrics for each associated station. In oneembodiment, the MAC processor also maintains the metrics on a set ofpresently unassociated but previously associated stations.

Again, while one application described is ascertaining at a stationwhether the station associates or reassociates with an access point, analternate embodiment is applicable to ad-hoc communication: ascertainingat a station whether the station communicates with another station on anad-hoc basis.

Selecting the Initial Rate

In one embodiment, when a station receives a packet from a remotestation, the latest EVM from the remote station is used to select theinitial rate for communicating with that remote station.

The MAC processor 214 includes a memory that stores a table of datarates and the EVM range for such a data rate.

In the case that the remote station successfully receives the responsetransmitted at the initial selected rate, communication continues atthat rate. In the case that the remote station does not successfullyreceive the response, the initial rate is lowered until communication issuccessful.

Compared to the prior art method of iterating starting at the highestrate, fewer iterations should be needed before either successfulcommunication is achieved, or it is ascertained that no communication ispossible.

ALTERNATE EMBODIMENTS Access Point Selection According to a Plurality ofMetrics

In another embodiment, the set of criteria for selecting an access pointfor association is according to a measure of the EVM between the stationand the remote station, and also from one or more other metricsavailable to the MAC. In one embodiment, one such metric is the packeterror rate (PER) for packets from the remote station. For example, in anEVM-capable station, if the EVM from a remote station that is acandidate for association indicates a high signal quality, but the PERfor packets for packets received is high, e.g., indicating a poorreceiver, the station may determine that the maximum supported data rateis lower than that indicated only by the EVM. This may lead to selectinga different access point for association.

In another embodiment, one of the other metrics used is thecarrier-to-noise ratio (CNR) at the PHY. U.S. patent application Ser.No. 10/698,703 titled “INITIAL TIMING ESTIMATION IN AN WIRELESS NETWORKRECEIVER to inventors Hart et al., filed Oct. 31, 2003, Attorney/AgentDocket No. CISCO-7702, and assigned to the assignee of the presentinvention, describes how the CNR of a packet may be estimated from anaverage power measure determine from samples of the received data aftera start-of-packet indication and from an average power measure determinefrom samples of the received data before such a start-of-packetindication. Such U.S. patent application Ser. No. 10/698,703 isincorporated herein by reference.

In yet another embodiment, one of the other metrics used is thebit-error-rate (BER) at the receiver.

In yet another embodiment, the receiver includes a filter in the receivepath, and provides a measure of the RSSI both pre-filter and postfilter. Comparing the pre- and post-filter signal strengths provides ameasure of the amount of adjacent channel interference. See U.S. patentapplication Ser. No. 10/622,175 titled “ADAPTIVE AGC IN A WIRELESSNETWORK RECEIVER,” filed Jul. 17, 2003 to inventors Adams, et al.,Attorney/Agent Docket No. CISCO-7343, and assigned to the assignee ofthe present invention, describes such a receiver. In yet anotherembodiment, one of the other metrics used is a comparison of thein-channel carrier to adjacent channel interference.

In one embodiment, the EVM (possibly in combination with one or moreother metrics) is used not only to select the data rate for transmittingto a remote station, but also to request the remote station send packetsat a rate determined according to the EVM of signals received from thatstation.

In one embodiment, both the EVM and the RSSI of signals received fromthe remote station are passed to the MAC and examined. If the EVMindicates a low signal quality, but the RSSI indicates high signalpower, the station can decide that this particular remote stationtransmits signals of low quality, even though the link may be good. Thestation can then decide whether or not to associate with such a remotestation.

Sending Signal Quality Information to the Remote Station

Thus, a method and apparatus has been described for selecting an accesspoint for association according to a measure of the EVM of signals to orfrom the access point.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to one of ordinary skill in the art from this disclosure, inone or more embodiments.

Similarly, it should be appreciated that in the above description ofexemplary embodiments of the invention, various features of theinvention are sometimes grouped together in a single embodiment, figure,or description thereof for the purpose of streamlining the disclosureand aiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the Detailed Description are hereby expressly incorporatedinto this Detailed Description, with each claim standing on its own as aseparate embodiment of this invention.

“Variants of the IEEE 802.11 standard” as used herein means the variantsand proposed variants of the IEEE 802.11 standard. Variants are versionsdefined in clauses of the standard and proposed amendments of thestandard.

It should be appreciated that although the invention has been describedin the context of variants of the IEEE 802.11 standard that use OFDMpackets, the invention is not limited to such contexts and may beutilized in various wireless network applications and systems, forexample in a system that uses packets other than OFDM packets, e.g., theIEEE 802.11b standard, or in a network that conforms to a standard otherthan IEEE 802.11. Furthermore, the invention is not limited to any onetype of architecture or protocol, and thus, may be utilized inconjunction with one or a combination of other architectures/protocols.For example, the invention may be embodied in transceivers conforming toother standards and for other applications, including other WLANstandards, Bluetooth, GSM, PHS, CDMA, and other cellular wirelesstelephony standards.

While one embodiment of the OFDM receiver (FIG. 3) accepts an IF signalthat requires further down-conversion to obtain baseband I,Q signals, inanother embodiment, the analog portion provides baseband I,Q signalsthat require no further down-conversion.

While one embodiment of the station (FIG. 2) is for half-duplexoperation, and includes a transmit/receive switch 204, other embodimentsare for full duplex operation.

The phrase “an EVM calculator that determines a measure of the EVM” andsimilar phrases include the case of EVM calculator 229 described abovethat determines in hardware the numerator of an expression a function ofthe calculated EVM and passes the numerator and denominator to the MACprocessor so that no division is carried out by the EVM calculator 229itself.

While the embodiments above use an EVM calculator that determines ameasure of the EVM that is proportional to the square of the calculatedEVM, other embodiments may use other measures of the EVM. All such othermeasures of the EVM, so long as they are monotonic functions of anapproximation of the EVM, are within the scope of the invention.

While embodiments above use an EVM calculator that excludes the pilottones, other embodiments may use a measure of the EVM that includes thepilot tones.

Thus, while there has been described what is believed to be thepreferred embodiments of the invention, those skilled in the art willrecognize that other and further modifications may be made theretowithout departing from the spirit of the invention, and it is intendedto claim all such changes and modifications as fall within the scope ofthe invention.

1. A method comprising: wirelessly receiving data corresponding to apacket in a wireless station from at least one remote station, thewireless station including an analog-to-digital converter and an errorvector magnitude (EVM) calculator configured to determine an EVMmeasure; sampling the receiver data to provide samples of the receiveddata; determining the EVM measure from the samples of the received datausing the EVM calculator; and selecting one of the remote stations forcommunication according to one or more criteria including the respectivedetermined EVM measure.
 2. A method as recited in claim 1, wherein theselected remote station is an access point, the method furthercomprising associating with the selected remote station as a clientstation of the access point.
 3. A method as recited in claim 1, whereinthe selecting selects one of the remote stations for communication on anad-hoc basis.
 4. A method as recited in claim 1, wherein the determiningof the EVM measure includes determining a function distances of datapoints to respective approximations of ideal constellation points.
 5. Amethod as recited in claim 1, further comprising demodulating at leastpart of the packet, and wherein the determining of the EVM measureincludes using the demodulated at least part of the packet to determinea function distances of data points to respective ideal constellationpoints.
 6. A method as recited in claim 1, further comprising: selectingthe data rate for communicating with the selected remote stationaccording to at least the determined EVM measure.
 7. A method as recitedin claim 6, wherein the selecting the data rate for communicating to theremote station is according to the determined signal quality and atleast one other metric.
 8. A method as recited in claim 7, wherein theat least one other metric includes the packet error rate for packetsfrom the remote station.
 9. A method as recited in claim 1, furthercomprising: in the case that the received data is ascertained to be aprobe request or an association request for association orre-association, transmitting a packet to the particular remote stationthat sent the probe request or association request, the transmittedpacket including an indication of the determined EVM measure of thepacket from the particular remote station.
 10. A method as recited inclaim 1, wherein one of the remote stations is an access point withwhich the station is associated, such that selecting one of the accesspoints for association includes deciding whether or not to roamaccording to the EVM from data received from the remote access pointwith which the station is associated.
 11. A method as recited in claim1, wherein the received data is orthogonal frequency divisionalmultiplex (OFDM) data.
 12. A method as recited in claim 1, wherein thedetermining of the measure of the EVM of the received packet includesdetermining the average of the squared Euclidian distance on the I,Qplane between decision-point samples of the signal received and thenearest ideal constellation points to the decision point samples.
 13. Amethod as recited in claim 1, wherein the determining of the measure ofthe EVM of the received packet includes determining the average of thesquared Euclidian distance on the I,Q plane between decision-pointsamples of the signal received and the correct ideal constellationpoints for the signal as determined by demodulating the signal.
 14. Acomputer readable storage medium configured with instructions that whenexecuted by a processor cause carrying out of a method comprising:wirelessly receiving data corresponding to a packet in a wirelessstation from at least one remote station, the wireless station includingan analog-to-digital converter and an error vector magnitude (EVM)calculator configured to determine an EVM measure; sampling the receiverdata to provide samples of the received data; determining the EVMmeasure from the samples of the received data using the EVM calculator;and selecting one of the remote stations for communication according toone or more criteria including the respective determined EVM measure.15. A computer readable storage medium as recited in claim 14, whereinthe selected remote station is an access point, the method furthercomprising associating with the selected remote station as a clientstation of the access point.
 16. A computer readable storage medium asrecited in claim 14, wherein the determining of the EVM measure includesdetermining a function distances of decision-point samples to respectiveapproximations of ideal constellation points.
 17. A computer readablestorage medium as recited in claim 14, further comprising: selecting thedata rate for communicating with the selected remote station accordingto at least the determined EVM measure
 18. An apparatus comprising: awireless receiver configured to wirelessly receive data corresponding toa packet; an analog-to-digital converter configured to produce sampleddata; an error vector magnitude (EVM) calculator configured to determinean EVM measure from the sampled data; a controller coupled to the EVMcalculator and configured to select, in response to the wirelessreceiver data a corresponding to packets from one or more remotestations, one of the remote stations for communication according to oneor more criteria including the respective determined EVM measure.
 19. Anapparatus as recited in claim 18, further comprising a wirelesstransmitter configured to transmit, wherein the controller is furtherconfigured to transmit, in the case that the received data isascertained to be a probe request or an association request forassociation or re-association, a packet to the particular remote stationthat sent the probe request or association request, the transmittedpacket including an indication of the determined EVM measure of thepacket from the particular remote station.
 20. An apparatus as recitedin claim 18, wherein the EVM calculator is configured to determine afunction distances of decision-point samples to respectiveapproximations of ideal constellation points in order to determine theEVM measure includes determining.
 21. An apparatus as recited in claim18, wherein the data corresponding to a packet includes OFDM data.